GNU Profiler - gprof

gprof - How it Works

How does it work?

• Compiler inserts hooks into each function ⟶ counts number of calls

• Signal handler runs periodically to gather statistic information about each call

Compiler and Linker Calls

$ gcc -pg -c -o program.o program.c
$ gcc -pg -g -c -o program.o program.c # debug info
$ gcc -pg -o program program.o

• Running program creates a file gmon.out in the current working directory

• Interpreted by gprof

Using gprof

Basic Usage

$ gprof program gmon.out
...

Prints plenty of information

• Flat profile: (sorted) list of functions and their numbers. Good to initially find out about the hot spots.

• Call graph: node-by-node listing of call graph

• Explanations of both (suppress with --brief)

gprof: Flat Profile

$ gprof --brief --flat-profile program gmon.out
  %  cumulative  self             self    total
 time  seconds  seconds    calls us/call us/call name
 89.96     1.02    1.02 38000000    0.03    0.03 contains
  9.80     1.13    0.11  1000000    0.11    1.10 find_duplicates

• % time: percentage of entire runtime, including called subroutines

• cumulative seconds: same in seconds

• self seconds: time consumed by the function alone. The most valuable information ⟶ primary sorting criterion

• calls: total number of calls

gprof: Call Graph

$ gprof --brief --graph program gmon.out
index % time    self  children    called     name
...
-----------------------------------------------
                0.11    0.99 1000000/1000000     main [1]
[2]     97.6    0.11    0.99 1000000         find_duplicates [2]
                0.99    0.00 37000000/38000000     contains [3]
-----------------------------------------------
...

• The index line is the center (what the node is about)

• Lines above: callers

• Lines below: callees

  • 3/291: 291 total calls, 3 attributed to one particular caller

gprof: Interpreting The Results

1. Identify hot spots

   • Flat profile gives the hot spots ⟶ self seconds is the primary criterion

   • Scripting always possible if more is wanted

2. Call graph starting at the hot spots

$ gprof --brief --graph -f contains program gmon.out
...
index % time  self children    called       name
              0.00   0.00 1000000/38000000   main (6)
              0.00   0.00 37000000/38000000   find_duplicates (2)
[1]      0.0  0.00   0.00 38000000       contains [1]
...

gprof: Visualization

One wishes that can be visualized We have no such luck $ gprof program gmon.out | \ gprof2dot | \ dot -Tjpeg | \ display -